1. Field of the Invention
The present invention relates to a facsimile apparatus which is capable of transmission and reception of image signals in at least two communication standard modes, and more particularly, it concerns a facsimile apparatus which is capable of transmitting/receiving and printing image signals having a pixel density conforming to either the G3 or G4 standard.
2. Description of Related Art
Recently, the standardization of facsimile apparatuses has been promoted under the recommendation of the CCITT. There are four standards, i.e., Groups 1 to 4 (hereinafter referred to as G1 to G4), at present. Each standard employs its own protocol and coding scheme which is different from the others. G1, G2 and G3 facsimile machines are analog machines mainly used for communication over a public telecommunication network, while a G4 facsimile machine is a digital machine used for communication over a digital communication network. Accordingly, communication images are processed in different ways in analog and digital machines: in a G3 facsimile machine, image signals are in general MH (modified Huffman) coded, while a G4 facsimile machine employs the MMR (modified MR) coding scheme as the standard coding scheme. Further, the pixel density of communication images in analog and digital machines is different: in a G3 facsimile machine, the pixel density is represented by a metric system of units, like 8 dots/mm in the horizontal direction and 3.8 dots/mm in the vertical direction, while in a G4 facsimile apparatus the pixel density is represented by an inch system of units, like 200 to 400 dots/inch in both the vertical and horizontal directions.
Currently, the mainstream of facsimile communication takes place in the G3 standard which employs analog lines (public telecommunication lines) for communication. However, the growing increase in the amount of information being communicated has accelerated the adoption of G4 facsimile machines that employ digital communication lines. This transition from G3 facsimile machines to G4 facsimile machines necessitates the use of facsimile machine which meet both G3 and G4 standards.
FIG. 10 is a block diagram of such a facsimile apparatus which meets both the G3 and G4 standards. In this facsimile apparatus, a subject document or the like is read optically and is converted to an electrical signal by an image reader 50. The pixel density of the image signal read is converted by a pixel density converter 51, and the resultant image signal is sent to an encoder 52 where it is compressed. The compressed image data is stored in an image memory 53. When the data is to be transmitted to a G4 facsimile machine, the image data from the image memory 53 is MMR coded by an encode/decode unit 54, and the coded data is sent out on a communication line 61 by a G4 communication control unit 55. For communication to a G3 machine, the image data from the image memory 53 is MH coded by an encode/decode unit 56, and the converted MH codes are sent out on a communication line 62 by a G3 communication control unit 57.
Conversely, the G4 standard image data received by the communication control unit 55 is decoded by the encode/decode unit 54, the decoded data being stored in the image memory 53. The G3 standard image data received by the G3 communication control unit 57 is decoded by the encode/decode unit 56, and the decoded data is stored in the image memory 53. The image data from the image memory 53 is decoded by a decoder 58 for the purpose of expanding the image data. The pixel density of the decoded image data is converted by a pixel density converter 59, and the resultant image data is recorded by an image recorder 60 such as a thermal printer.
However, the pixel density of the G3 standard image data is different from that of the G4 standard image data. It is now assumed that the pixel density of both the image reader 50 and the image recorder 60 is fixed at, for example, 200 dots/inch in both the vertical and horizontal directions. In a case where image data with a pixel density of 200 dots/inch is received by the G4 communication control unit 55, the image reproduced by the recorder 60 corresponds to the original document because the pixel density of the received image data is the same as that at which the image data was recorded by the image recorder 60. However, in a case where G3 standard image data is received from the analog line, the image reproduced by the image recorder 60 is reduced or enlarged relative to the original document in the vertical and horizontal directions. More specifically, image data in the inch system (G4 standard, with a pixel density of 200.times.200 dots/inch.sup.2) corresponds to image data in the metric system (G3 standard) having a pixel density of 8.times.7.7 dots/mm.sup.2. However, 200.times.200 dots/inch.sup.2 is equivalent to about 7.87.times.7.87 dots/mm.sup.2. In consequence, the image in the metric system reproduced by the image recorder 60 is reduced in the longitudinal direction and is enlarged in the lateral direction.
FIG. 11A shows an image of an original 65 read according to the inch system (the G4 machine) and reproduced according to the metric system (the G3 machine). In this case, reproduction 66 of the original 65 is reduced in the lateral direction and is enlarged in the longitudinal direction. Accordingly, conversion of the pixel density is conducted by the pixel density converter 51 and 59 for the purpose of eliminating this problem. For example, when data of 8.times.7.7 dots/mm.sup.2 conforming to the G3 standard is converted to 200.times.200 dots/inch.sup.2 data conforming to the G4 standard, one dot is deleted per 64 dots in the horizontal direction, and one row, having the same pixel values as those in the previous row, is added per 45 rows of pixels in the vertical direction.
This arrangement allows a reproduced image to be of the same size as that of the original document. However, deletion or interpolation of pixel data which are processed during the conversion of the pixel density deteriorates the quality of the image. This process is illustrated in FIG. 11B. A document 67 reproduced by the image recorder 60 at a converted pixel density has the same size as that of the original document 65 but its quality is degraded.